THE RESPONSES OF SALT MARSH CREEKS AND DITCHES WHEN EXPOSED TO A SPATIAL GRADIENT OF NITROGEN CONCENTRATIONS OVER AN INTERVAL OF DECADES

High nutrient levels can cause the thinning of salt marsh peat and a rapid increase to high nitrogen levels may cause the collapse of edges along salt marsh creek and ditches. What is unclear is whether high nitrogen levels and the resulting lower root to shoot ratios have an affect on the stability of banks of creeks and ditches over a period of decades or do the edges eventually stabilize. Using several sets of aerial photography covering the time interval 1956 to 2012, the long term trends for ditch and creek edges in a high nutrient bay was compared with trends on marshes surrounded with lower nitrogen levels. Records of nutrient concentrations for this estuary start in 1968 and indicate that high levels of nitrogen already existed at that time. In the high nutrient West Bay, some edge failure and blocks are possibly seen in 1956 photos but are infrequent and may represent advanced stages of edge failure at that time. Recent photos and on the ground inspection do not show the expected pattern of cracks and blocks that indicate edge failure. In high nitrogen locations, Spartina alterniflora has instead grown into many of the man made ditches. In locations where N concentrations are low and declining, the regrowth into ditches is not proceeding as rapidly and the widening of ditches is also occurring more extensively. In all locations the natural creeks have widened and show signs of meandering. Some spatial patterns of peat strength are also presented. It is concluded that these marsh edges exposed to high nitrogen concentrations may have reached an equilibrium. At least in these specific marshes, the higher rates of sediment capture reported in several fertilized marsh experiments seem to outweigh the weakening effects from thinning peat when viewed over time periods extending over several decades.